Aryl-substituted propionic acid esters, such as, methyl 3-(3,5-dialkyl-4-hydroxyphenyl)propionates used as antioxidants for plastics, rubber and other polymers, have been prepared by various methods in the prior art. For example, U.S. Pat. Nos. 3,247,240, 3,285,855 and 3,364,250 disclose preparing methyl 3-(3,5-dialkyl-4-hydroxyphenyl) propionates by reacting a 3,5-dialkyl-4-hydroxybenzene with an acrylate in the presence of a base catalyst, with or without a solvent. The addition of the methyl acrylate in the above processes is over a period of approximately 20 minutes, but the conversion rate is very slow, from about 6 to 72 hours in the presence of a solvent and at least 3 hours without a solvent.
U.S Pat. No. 3,840,855 is directed to a process for producing an alkyl ester of 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionic acid by reacting 2,6-di-t-butylphenol with an alkyl acrylate in the presence of a catalytic amount of a metal hydride with or without a solvent. As in the processes of the above-identified patents, the conversion rate is extremely slow, from about 28 to 42 hours to obtain less than 92% yield.
The process disclosed in U.S. Pat. No. 4,529,809 reacts a stoichiometric excess of an olefinic ester with a sterically hindered phenol in the presence of a base catalyst, with or without a solvent, wherein the reaction time ranged from 11 to 23 hours with reported yields of 32 to 99%.
U.S. Pat. No. 4,547,585 is directed to the formation of methyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, an intermediate product, by reacting an alkyl acrylate with 2,6-di-t-butylphenol in the presence of an alkaline catalyst and preferably a solvent, such as t-butyl alcohol In this process unreacted acrylate must be removed and the reaction time is from 2 to 10 hours.
In an attempt to minimize the formation of undesirable by-products, U.S. Pat. No. 4,228,297 discloses a process wherein the methyl acrylate is gradually added over a 2 hour period to the phenolic compound in the presence of an alkaline catalyst with or without an aliphatic alcohol or polar aprotic solvent. Preferably an aliphatic alcohol, such as isopropyl alcohol, is used. However, once all of the acrylate is added, an additional 3-4 hours of mixing is necessary to complete the reaction, and then the excess acrylate must be removed before acidifying the reaction mixture. The ester was reported in yields of 84% and 87%.
U.S. Pat. No. 4,659,863 discloses an improved process for preparing methyl esters of hindered phenol derivatives by reacting a hindered phenol with methyl acrylate in the presence of an alkaline catalyst and a reaction rate increasing portion of a solubilizing agent such as DMSO. The methyl acrylate can be added by rapid addition, which is stated to be from 15 to 60 min., to the reaction mixture and unreacted acrylate is removed after completion of the reaction.
U.S. Pat. No. 5,089,656 is directed to a process for preparing aryl substituted propionic acid esters in a shorter period of time with improved conversion, higher purity and minimum formation of undesirable by-products, by removing substantially all of the side-product prior to adding the complexing agent and adding all or substantially all of the acrylate at once to the reaction mixture. The disadvantage of this particular process is that it is highly exothermic. Hence, the temperature of the reaction must be monitored carefully so as to avoid the polymerization of the acrylate. The monitoring of the heat of the reaction is necessary not only to avoid the formation of an undersirable by-product, but more importantly to prevent the reaction from becoming a so-called "acrylate runaway" reaction. Since the acrylate polymerization reaction is also exothermic, it releases additional amounts of heat, which, in turn, causes even more of the acrylate to polymerize. If this additional heat is not dissipated, the reaction can get out of hand and turn into an acrylate "runaway" reaction.